Fixes the exception while decoding the stack dump:
File "tools/gdb_panic_server.py", line 270, in _respond_mem
result += "{:02x}".format(stack_data[addr - stack_addr_min])
ValueError: Unknown format code 'x' for object of type 'str'
This follows a similar approach as UART core dump handling in
idf_monitor. Panic handler message is detected in the output,
collected into a file, and the file is passed to the decoding script.
In this case, the decoding script acts as a tiny GDB server, so we
can ask GDB to perform the backtrace.
